The present invention relates to a process for the preparation of zeolitic catalysts, comprising zeolite and oligomeric silica, which can be used in industrial reactors.
More specifically, the invention relates to a process for the preparation of zeolitic catalysts characterized by an original morphology which consists in directly mixing the suspension resulting from the synthesis of the zeolite with an oligomeric silica sol and in subjecting the mixture to rapid drying.
The invention also relates to the materials obtained by means of the above process and to the processes in which these are used as catalysts.
Zeolites and zeolitic materials are known in literature as basic components for the preparation of catalysts, which can be used in numerous reactions of industrial interest.
For example, zeolites of the MFI type with a low content of trivalent hetero-elements, are known in literature as base material for the preparation of catalysts which can be used in the transposition reaction of oximes to amides (EP 242,960).
Zeolites of the MFI type, in which the hetero-element is Ti (Titanium-Silicalites TS-1) are known as materials for the preparation of catalysts which are used in many oxidation reactions, among which ammoximation reactions (U.S. Pat. Nos. 4,410,501; 4,794,198).
These materials however, if forming the only active component, have limited possibilities for use in industrial reactors.
In fact, whereas, on the one hand, the small dimensions of the zeolite crystals favour the intraparticle diffusion of the reagents and reaction products and allow good catalytic performances to be obtained, on the other hand similar dimensions can prevent interparticle diffusion in fixed bed reactors, or the separation of the zeolitic catalyst from the reaction medium in mixing reactors.
An increase in the dimensions, as also the resistance to friction and tensile strength, is generally obtained by combining the zeolitic material with compounds of an inorganic nature (ligands) in the forming phase.
The methods for the preparation of bound zeolites must be such as not to cause blockage of the zeolitic cavities, which would obviously create a reduction in the catalytic activity.
For reactions in which a catalytic contribution on the part of the ligand should be avoided, as for example in many oxidations and acid-catalyzed reactions, the use of a ligand of a catalytically inert material such as silica, is of particular interest.
Patent EP 265,018, for example, describes a process for preparing zeolitic catalysts based on the rapid drying of an aqueous dispersion consisting of crystalline zeolite, oligomeric silica and tetra-alkylammonium hydroxide.
The process, however, is complex and comprises a series of steps among which the separation of the zeolite crystals obtained from the hydro-thermal synthesis and their subsequent washing before dispersion in the aqueous solution of oligomeric silica previously formed by the hydrolysis of a tetra-alkyl ortho silicate in the presence of a tetra-alkylammonium hydroxide.
European patent application EP 906,784 discloses a simplified process for preparing catalysts comprising zeolites and oligomeric silica, which avoids the steps relating to the separation of the crystals and subsequent washing.
In practice, a tetra-alkyl ortho silicate compound is hydrolyzed directly in the suspension resulting from the synthesis of the zeolite, containing zeolite crystals and the residual templating agent (tetra-alkylammonium hydroxide).
The slurry obtained is then subjected to rapid drying by feeding to a spray-dry.
To avoid the separation step of the zeolite, a great advantage, from an industrial point of view, is mainly when operating with zeolite crystals having dimensions of less than 0.5 xcexcm.
In this case, the crystals cannot be separated from the synthesis medium with the usual techniques, such as filtration or with continuous centrifugations, but require the use of more expensive techniques which operate batch-wise.
A new simplified process has now been found, which allows the preparation of zeolitic catalysts characterized by an original morphology and suitable for industrial use.
In particular, an object of the present invention relates to a process for the preparation of zeolitic catalysts based on the rapid drying of an aqueous dispersion comprising crystalline zeolite, oligomeric silica and tetra-alkylammonium hydroxide, characterized in that the aqueous dispersion is prepared by directly mixing the suspension resulting from the synthesis of the zeolite, with an oligomeric silica sol obtained from the hydrolysis of a tetra-alkyl ortho silicate compound in the presence of tetra-alkylammonium hydroxide.
The process according to the invention differs from that described in EP 906,784, in the preparation of oligomeric silica which, in this case, is obtained by separately hydrolyzing a tetra-alkyl ortho silicate compound in the presence of tetra-alkylammonium hydroxide, whereas in the process of the known art, the oligomeric silica precursor is added directly to the suspension resulting from the synthesis of the zeolite.
Operating according to the process of the invention, catalysts are obtained in the form of microspheres having dimensions ranging from 5 to 300 xcexcm, characterized by an external crown of essentially amorphous silica, which encloses inside an essentially crystalline low density phase consisting of zeolite (FIGS. 1, 2).
This particular morphology has never been described in the catalysts of the known art, in which the zeolite is generally uniformly dispersed in the amorphous silica chase.
The microspheres have a high crushing strength and a weight ratio oligomeric silica/zeolite ranging from 0.05 to 0.7.
The binding amorphous phase is characterized by an essentially mesoporous pore distribution and high surface area.
The process of the invention can be successfully applied to crystallization slurry of zeolites in which the molar ratio H2O/SiO2 is within the range of 4-35, whereas the molar ratio H2O/SiO2 of the zeolites prepared according to the process described in EP 906,784 ranges from 10 to 35, as lower ratios cause problems of instability of the slurry to be fed to the spray-dry.
The use of more concentrated reagent mixtures increases the productivity during the synthesis phase of the zeolite and creates more concentrated slurries in the feeding to the atomizer (a parameter which is known to significantly influence the dimensions of the atomized product).
Zeolites which are particularly suitable for being bound according to the present invention are those of the MFI, MFI/MEL and MEL group selected from:
1) MFI zeolites having the formula
p HMO2xc2x7q TiO2xc2x7SiO2 
wherein M is a metal selected from aluminum, gallium and iron, p has a value of 0 to 0.04 and q has a value ranging from 0.0005 to 0.03.
In particular when p is 0, the zeolite is Titanium-Silicalite TS-1 described in U.S. Pat. No. 4,410,501; zeolites in which p is different from 0 and M=Al, Ga and Fe are described in EP 226,257, EP 266,825 and EP 226,258, respectively;
2) MFI zeolites having the formula
a Al2O3xc2x7(1xe2x88x92a) SiO2 
wherein a has a value ranging from 0 to 0.02.
In particular when a is 0 the zeolite is silicalite S-1 described in U.S. Pat. No. 4,061,724;
when a is different from 0 the zeolite is ZSM-5 described in U.S. Pat. No. 3,702,886 and in the new publication U.S. No. 29,948.
3) MEL or MFI/MEL zeolites having the formula
x TiO2xc2x7(1xe2x88x92x) SiO2 
wherein x has a value ranging from 0.0005 to 0.03.
These zeolites are described in BE 1,001,038 and are called TS-2 and TS-1/TS-2.
The binding phase consists of an amorphous mesoporous silica or silica-alumina synthesized as described in EP 340,868 and in EP 812,804.
It is therefore characterized by a high surface area and distributed pore volume in the mesoporous region.
This guarantees the absence of diffusion problems due to reactions typically catalyzed by zeolitic, i.e. microporous materials.
The active phase, which is enclosed in the crown of the amorphous ligand, fully preserves its specific characteristics, as its interactions with the binding phase are minimized.
The preparation of the zeolite according to the process of the invention takes place by the hydrothermal treatment at autogenous pressure, at a temperature ranging from 150 to 230xc2x0 C. and for a time ranging from 0.5 to 48 hours, being the alkaline metals absent, of a mixture containing a silicon source, a tetra-alkylammonium hydroxide (TAAxe2x80x94OH), and optionally sources of Ti and/or Al, Ga, Fe.
The mixture has the following composition expressed as molar ratios:
Ti/Si=0-0.03;
M/Si=0-0.04 wherein M can be selected from Al, Ga or Fe;
TAAxe2x80x94OH/Si=0.2-0.5
H2O/Si=4-35
The sources of silicon, titanium, aluminum, iron, gallium are those described in U.S. Pat. No. 4,410,501, EP 226,257, EP 266,825, EP 226,258.
The TAAxe2x80x94OH source is selected from those described in BE 1,001,038.
The binding phase of the process according to the present invention is prepared by mixing a silica source, optionally an aluminum source, a tetra-alkylammonium hydroxide, at a temperature ranging from 20xc2x0 to 120xc2x0 C. and for a time ranging from 0.2 to 24 hours.
The molar composition of the mixture is the following:
TAAxe2x80x94OH/SiO2=0.04-0.40
H2O/SiO2=10-40
Al2O3/SiO2=0-0.02
The sources of silica, aluminum, tetra-alkylammonium hydroxide are those described in EP 340,868 and in EP 812,804.
The alcohols present in the reagent mixture are those deriving from the hydrolysis of the reagents selected.
The H2O/SiO2 ratio=10-40 is selected so as to obtain a sol after the hydrolysis of the reagents.
The sol resulting from the preparation of the oligomeric silica is added to the crystallization slurry obtained from the preparation of the zeolite.
A plasticizing compound, such as for example, polyvinyl alcohol or methyl cellulose, can be optionally added in this phase.
The relative quantities of oligomeric silica sol and slurry, deriving from the crystallization of the zeolite, to be mixed together, are calculated so as to have a weight ratio oligomeric silica/zeolite ranging from 0.05 to 0.70.
The resulting slurry is treated under stirring at a temperature ranging from 25xc2x0 C. to the boiling point of the mixture, for a period of time ranging from 1 to 48 hours. Preferably: 50-70xc2x0 C., 1-6 hours.
The slurry, resulting from the mixing of the oligomeric silica with the zeolite, is subjected to rapid drying by means of spray-dry and the product obtained is calcined.
The atomization tests were carried out with Niro Mobile Minor HI-TEC spray, feeding with a 1.5 mm nozzle.
The form and dimensions of the catalysts produced are determined by means of SEM (Scanning Electron Microscopy) analysis on sections of samples.
The samples obtained can be used as catalysts, in particular for fluid bed applications.
When the zeolitic phase is silicalite-1 (MFI with only a silica composition), the catalyst formulated in accordance with the method object of the present invention can be successfully used in the catalytic transposition reaction of oximes to amides, such as the Beckmann catalytic transposition (transformation of cyclohexanone-oxime to xcex5-caprolactam).
In fact, the formulation method used allows the properties of the active phase to remain unaltered.